This invention relates generally to the field of navigation systems which use a constellation of earth-orbiting satellites to determine the relative position between two users located on or near the earth's surface. More specifically, the invention depicts a method and apparatus for improving the accuracy of relative position estimates in such a satellite-based navigation system. It also relates to the field of collision avoidance as the knowledge of such accuracy is a key factor in preventing mid-air and ground collisions. Several satellite-based navigation systems are currently used by the worldwide community. For ease of discussion, the following disclosure will specifically focus on the NAVSTAR Global Positioning System (GPS) operated and maintained by the Department of Defense. This system, which is used for navigation, position determination and time-transfer applications, consists of a 24-satellite constellation. Each of them radiates precisely timed signals coded so that a receiver on or near the earth's surface can determine both the transmission time delay (or equivalently, distance) from the satellite to the receiver and the precise satellite position. By simultaneously receiving such signals from at least four satellites, the receiver can determine its position and time.
However, several sources or errors adversely affect the accuracy of such determination. These errors are primarily due to:
satellite ephemeris uncertainties (lack of precision in the satellite position on its pre-determined orbit) PA1 selective availability (deliberate degradation of the signals by the Department of Defense for non military users) PA1 ionospheric and tropospheric propagation delays PA1 satellite and receiver clock drifts PA1 multipath (multiple reflexion and scattering of a signal) PA1 receiver noise.
Although the differentiation of two nearby receivers' absolute positions (i.e., calculated independently one from the other) enables one to partly eliminate some of these errors, this type of post-processing, abundantly shown in the prior art, is by essence unable to attain the accuracy of a direct relative positioning method, cancelling out at the source errors common to both receivers. Accordingly, what is needed is a new method solving directly for the relative position of two users so that the accuracy of such position is improved.